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Wistow, Graeme wistow at FCRFV1.NCIFCRF.GOV
Fri Sep 27 13:38:00 EST 1991

Concerning the recent discussion about molecular clocks -

Another point to consider is that the products of homologous genes in different
species may acquire additional functions without gene duplication.

This has occurred for several enzymes which have been recruited as bulk
structural components of the lens in different vertebrate lineages. The first
example of this was LDH-B which in many birds and crocodiles also serves as
epsilon-crystallin (Wistow et al, Nature, 1987, 326, 622) and may account 
for almost half the total protein in the lens (Wistow etc, PNAS, 1990,87,6277).
There is a single gene for "both" proteins (Hendriks etc, 1988, PNAS, 85,7114). 
With two interesting exceptions, those species which use e-crystallin have 
acquired specific amino-acid sequence changes in LDH-B, presumably selected 
for the lens structural role of the protein.  
Allan Wilson (TIBS, 1987, 3, 241) noted that this probably explained his 
observations of a "markedly uneven rate" of change in LDH-B in birds, made 
over 20 years earlier.

Graeme Wistow
graeme at mge2.nei.nih.gov

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